1. Field of the Invention
The present invention relates to a motor driving apparatus which is suited to driving of a polyphase motor.
2. Description of the Background Art
FIG. 22 is a block diagram showing the structure of a conventional motor driving apparatus as a background of the present invention. This apparatus 90 is used to drive a motor 201 which is a sensor-less (having no Hall element for detecting the rotating position) and brush-less three-phase motor. This apparatus 90 includes an output circuit 101, a selecting circuit 102, an induced voltage detecting circuit 108, a commutation control circuit 120, a starting circuit 121, terminals SU, SV, SW, and a terminal scom. To drive the motor 201 by using the apparatus 90, the terminals SU, SV, SW are connected to the three coil terminals of the motor 201 and the terminal scom is connected to the neutral terminal of the motor 201.
The output circuit 101 has a plurality of switch elements (not shown) interposed between the terminals SU, SV, SW and a power-supply line (not shown) and a ground line. These plurality of switch elements are selectively turned on (conduct) and off (disconnect) to realize a plurality of current-supply patterns. The induced voltage detecting circuit 108 is connected to the terminals SU, SV, SW and the terminal scom to detect points at which the induced voltages at the terminals SU, SV, SW cross the neutral voltage of the motor 201 inputted through the terminal scom.
The commutation control circuit 120 generates and outputs a control signal A1 for controlling on/off operation of the plurality of switch elements in the output circuit 101 on the basis of a detection signal B2 outputted from the induced voltage detecting circuit 108 when the motor 201 is operating or when the rotor is rotating. The control signal A1 is sequentially switched in accordance with the electrical angle of the rotor between a plurality of values corresponding to the plurality of current-supply patterns. When the motor 201 is operating, the selecting circuit 102 sends the control signal A1 to the plurality of switch elements in the output circuit 101 as control signals C1 to C6. The motor 201 is thus supplied with current in the plurality of current-supply patterns in accordance with the electrical angle of the rotor.
The starting circuit 121 outputs a control signal A2 when starting the motor 201, that is, when the rotor at rest starts rotating. When the motor 201 starts rotating, the selecting circuit 102 selects the control signal A2 and sends it to the output circuit 101 as the control signals C1 to C6. Thus torque for starting is given to the rotor and the rotor starts rotating. When the operation of starting the motor 201 is finished, the selecting circuit 102 selects the control signal A1 and sends it to the output circuit 101, as stated above.
FIG. 23 is a timing chart showing signals in individual parts of the apparatus 90. In the apparatus 90, when starting the motor 201, the starting circuit 121 sequentially forces the current-supply pattern to change independently of the position of the rotor (rotating position) in a given period from the beginning of starting (to the point P in FIG. 23) to give starting torque to the rotor. As shown in FIG. 23, the control signals C1 to C6 (equivalent to the control signal A2) in the starting period from the beginning of starting to the point P change in the same order as the control signals C1 to C6 (equivalent to the control signal A1) in the driving period after the point P, with their switching time intervals gradually becoming shorter.
That is to say, in the starting period, the current-supply patterns corresponding to the range from 0 to 360.degree. in electrical angle of the rotor in the driving period are generated irrespective of the position of the rotor. In this way, in the conventional motor driving apparatus, the current-supply patterns for starting are developed in a predetermined order irrespective of the stop position at which the rotor of the motor 201 rested before started, so that the rotor may once reversely rotate and then normally rotate when started.
Further, as shown in FIG. 23, in the driving period in which the rotor rotates, spike voltage appears due to switch of the current-supply pattern (i.e. commutation switch) in the induced voltages SU, SV, SW (hereinafter the terminal voltages are simply represented by the same characters as the terminals) induced at the terminals SU, SW, SW (for example, the part surrounded by the dotted circle in FIG. 23). As has been already stated, the induced voltage detecting circuit 108 detects the points at which the induced voltages SU, SV, SW cross the neutral voltage scom (the parts surrounded by the solid circles in FIG. 23) and outputs the detection signal B2. Then the commutation control circuit 120 sequentially switches the control signal A1 between a plurality of kinds corresponding to the plurality of current-supply patterns on the basis of the detection signal B2. Accordingly the spike voltage may cause the control signal A1 to be switched by erroneous timing.
For the purpose of avoiding such erroneous detection in the induced voltage detecting circuit 108, a mask circuit is provided to prevent the induced voltage detecting circuit 108 from detecting crossing of the induced voltages SU, SV, SW and the neutral voltage scom in the vicinities of spike voltage (mask period). However, the mask period is set to a given length of time for a structural reason of the mask circuit, and therefore the following problems arise. When the rotor of the motor 201 rotates at low speed, the ratio of the mask period to one-turn period is so small that the erroneous detection cannot be prevented sufficiently. When it rotates at high speed, the ratio of the mask period becomes unnecessarily large to possibly prevent normal detection. That is to say, it has been difficult to prevent such erroneous detection at a wide range of rotating speeds.
Also, there is another known motor driving apparatus improved to switch the current-supply pattern not instantaneously but softly in a given time width to reduce acoustic noise caused by switch of the current-supply pattern. However, this improved apparatus realizes the slanted switching by utilizing charge/discharge of a capacitance element, so that the time width for switching remains constant independently of the rotating speed. Accordingly, when the rotor rotates at high speed, the ratio of the time width for switching to one-turn period becomes large, leading to the problem that sufficiently high power cannot be obtained.
Moreover, this improved apparatus has the problem that it cannot make PWM control of the output current. A conventionally known type of motor driving apparatus makes the switching elements in the output circuit 101 perform pulsing operation on the basis of the PWM control to control the effective output current, thereby enabling versatile control of power of the motor 201. However, the above-mentioned apparatus improved to reduce acoustic noise cannot be adapted for PWM control. In other words, a motor driving apparatus performing PWM control cannot reduce acoustic noise.